708 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



form of fissure eruptions or in the form of central eruptions. The writer believes 

 that a third method should be entertained as a possibility, namely, by the partial 

 or complete foundering of batholithic roofs. The relation of each of these three 

 phases of volcanic action to abyssal injection may now be outlined. 



Fissure Eruptions. — The regional lava floods, known to have emanated from 

 simple fissures in their underlying terranes, range in date from the pre-Cambrian 

 to the present. As we have seen, they are, without exception, of basaltic com- 

 position. Such magma must be exotic, that is, it cannot be explained as due 

 to the refusion of material from the earth's acid or sedimentary shells. It is 

 the only lava (extrusive magma) to which a secondary origin cannot theoretically 

 be ascribed. But the very low original slopes of the flows (very often inclined 

 at much less than one degree from the horizontal plane) and their great lengths 

 show that the basalt of fissure eruptions is notably superheated. Such tempera- 

 ture is appropriate to assimilation of foreign rock. That the solution of pre- 

 Cambrian gneisses or of other rocks has not taken place in sensible amount 

 must have either of two meanings. It may mean that the various abyssal injec- 

 tions underlying the lava field are narrow, with widths to be measured in feet 

 or tens of feet, but not in many thousands of feet. Or failure to assimilate 

 may be due to special rapidity of injection, with simultaneous extrusion; for 

 solution of foreign rock must take considerable time. The observed average 

 size of the feeding channels (dikes) in the great lava fields of the western 

 United States, of northwestern Europe, of India, and other regions corresponds 

 with the former conclusion. The vast Icelandic flow of 1783 and the nature of 

 the individual flows in every pre-historic lava field show or at least suggest that 

 each extrusion has been rapid. The controlling condition for the lack of assimi- 

 lation is probably the narrowness of the abyssal injections, at least in the part 

 traversing the sedimentary and acid shells of the earth. 



The fissures need not be planes of strong, or even discernible faulting. 

 For example, the Purcell Lava, covering many thousands of square miles, issued 

 quietly from many cracks in the Siyeh-Kitchener sea-bottom, and covered the 

 muds and sands with a continuous sheet of basalt, which evidently flowed on a 

 flat, practically unbroken surface. This eruption illustrates, in fact, the very 

 common association of fissure eruption with downwarps of the broad, gentle, 

 geosynclinal order. Whether the down-warping is the effect of abyssal injection, 

 as suggested by the writer in a published paper,* or whether the abyssal injection 

 and surface outflow are the effect of down-warping, are important questions 

 which will not be discussed in this place. 



The effusion of a basaltic flood is generally ascribed to the mere squeezing 

 out of the magma from beneath a cracked and sinking Earth-crust. Yet some 

 force may also be available from the expansion of the substratum material as it 

 rises to levels of greatly lessened pressure. This expansion is of two kinds — 

 that of the lava regarded as gas-free, and that of the gases separated from it 

 in bubble form. If the expansional energy of the liquid proper is not all used 

 up in driving asunder the walls of the injected body, some of that great force 



* E. A. Daly, American Journal of Science, Vol. 22. 1906, p. 195. 



